Effect of the Pelleting Process on Diet Formulations with Varying Levels of Crystalline Amino Acids and Reducing Sugars on Digestibility in Growing Pigs

The objective of this study was to determine the effect of thermal processing on the digestibility of amino acids (AA) in diets with or without increased concentrations of free amino acids and reducing sugars (RS). To measure AA digestibility, a total of eight individually housed barrows (initially 69.2 ± 6.8 lb) that had a T-cannula installed in the distal ileum were allotted to a replicated 8 × 8 Latin square design with 8 diets and eight 7-d periods. Thus, each pig was fed each diet in one period and no pig received the same diet more than once. Each period lasted 7 days with the initial 5 days being the adaptation period, and ileal digesta was collected for 9 hours on d 6 and 7. Treatments were arranged in a 2 × 2 × 2 factorial with main effects of crystalline AA concentration (low vs. high), reducing sugars (low vs. high), and diet form (mash vs. pellet). There was no feed form × crystalline AA inclusion × RS inclusion interaction for standardized ileal digestible (SID) AA. There was a feed form × RS interaction (P < 0.026) for SID tryptophan. Feeding pelleted high RS diets resulted in decreased SID of tryptophan compared with mash high and low RS diets, and pelleted low RS diets. For the main effects of feed form, the SID of total AA, crude protein (CP), and indispensable AA increased (P < 0.042) in pigs fed pelleted diets compared with those fed mash diets. For the main effects of crystalline AA inclusion, pigs fed low or high crystalline AA inclusion had similar SID of total AA and CP. Pigs fed high crystalline AA had increased (P = 0.007) SID of tryptophan compared with those fed low crystalline AA diets. The SID of lysine tended to increase (P = 0.076) in pigs fed high crystalline AA diets compared with those fed low crystalline AA inclusion diets. Pigs fed high crystalline AA had decreased (P = 0.050) SID histidine compared with those fed low crystalline AA diets. The SID of arginine and isoleucine tended to decrease (P < 0.079), in pigs fed high crystalline AA. In pigs fed high crystalline AA, the SID of serine and glycine decreased (P < 0.042) compared with those fed low crystalline AA. For the main effects of RS diets, pigs fed high RS diets had decreased (P < 0.05) SID of total AA, CP, indispensable AA, alanine, aspartic acid, cysteine, glutamic acid, and serine. In conclusion, there was no evidence of interactions between diet types. Therefore, pelleting diets with increased concentration of crystalline AA or RS at the conditions reported herein did not reduce the AA digestibility. However, pelleting diets resulted in improved AA digestibility. Diets formulated with increased concentrations of crystalline AA had increased SID of tryptophan. Formulating diets with 20% DDGS and 15% bakery meal (high RS) resulted in decreased AA digestibility compared with the cornsoybean meal-based diets.


Summary
The objective of this study was to determine the effect of thermal processing on the digestibility of amino acids (AA) in diets with or without increased concentrations of free amino acids and reducing sugars (RS). To measure AA digestibility, a total of eight individually housed barrows (initially 69.2 ± 6.8 lb) that had a T-cannula installed in the distal ileum were allotted to a replicated 8 × 8 Latin square design with 8 diets and eight 7-d periods. Thus, each pig was fed each diet in one period and no pig received the same diet more than once. Each period lasted 7 days with the initial 5 days being the adaptation period, and ileal digesta was collected for 9 hours on d 6 and 7. Treatments were arranged in a 2 × 2 × 2 factorial with main effects of crystalline AA concentration (low vs. high), reducing sugars (low vs. high), and diet form (mash vs. pellet). There was no feed form × crystalline AA inclusion × RS inclusion interaction for standardized ileal digestible (SID) AA. There was a feed form × RS interaction (P < 0.026) for SID tryptophan. Feeding pelleted high RS diets resulted in decreased SID of tryptophan compared with mash high and low RS diets, and pelleted low RS diets. For the main effects of feed form, the SID of total AA, crude protein (CP), and indispensable AA increased (P < 0.042) in pigs fed pelleted diets compared with those fed mash diets. For the main effects of crystalline AA inclusion, pigs fed low or high crystalline AA inclusion had similar SID of total AA and CP. Pigs fed high crystalline AA had increased (P = 0.007) SID of tryptophan compared with those fed low crystalline AA diets. The SID of lysine tended to increase (P = 0.076) in pigs fed high crystalline AA diets compared with those fed low crystalline AA inclusion diets. Pigs fed high crystalline AA had decreased (P = 0.050) SID histidine compared with those fed low crystalline AA diets. The SID of arginine and isoleucine tended to decrease (P < 0.079), in pigs fed high crystalline AA. In pigs fed high crystalline AA, the SID of serine and glycine Introduction Pelleted feed in swine diets is commonly used to improve feed efficiency, feed handling characteristics, and bulk density while decreasing feed wastage. Recent feed safety issues and the importance of pellet quality have led feed manufacture companies to steam condition the grow-finish pig diets at higher temperatures during the pelleting process. In addition, current trends in diet formulation in the swine industry have focused on increasing the use of crystalline amino acids (AA). Another common practice for swine nutritionists is to use byproduct ingredients to reduce feed costs. Common byproducts include distillers dried grains with solubles (DDGS) from the ethanol industry and bakery meal from the food and confectionary industry. Corn and soybean meal commonly used in finishing pig diets contain trace amounts of reducing sugars (RS). The reducing sugars glucose, sucrose, maltose, and fructose typically have concentrations of 0.66, 1.14, 0.23, and 0.4% in corn; 5.03, 4.91, 2.85, and 4.71% in bakery meal; and 1.84%, 0.19%, 2.28%, and 0.74% in DDGS. 4 Soybeans contain glucose ranging from 0.07 to 0.40% (DM-basis), however, glucose is destroyed in the crushing of soybeans to produce soybean meal. 5 The reducing sugars in feed ingredients may pose problems when pelleting diets at increased temperatures. Over-processing can potentially lead to a reduction in amino acid availability due to the Maillard reaction. The Maillard reaction is a non-enzymatic browning reaction between an amino group in amino acids, peptides, or proteins and a carbonyl group of reducing sugars such as glucose, fructose, or lactose. Variables optimizing the Maillard browning reaction include high temperatures and moisture levels, which can occur during feed processing. Thus, pelleting diets containing crystalline amino acids and co-products with reducing sugars may increase the risk for reduced amino acid availability and, consequently, impaired growth performance. Therefore, the objective of this experiment was to test the hypothesis that effects of pelleting swine diets containing free amino acids and reducing sugars at high temperatures will reduce the digestibility of amino acids.

Procedures
The University of Illinois Institutional Animal Care and Use Committee approved the protocol used in this experiment. This study was conducted at the University of Illinois Swine Research Center (Champaign, IL).
A total of eight individually housed growing barrows (initially 69.2 ± 6.8 lb) that had a T-cannula installed in the distal ileum were allotted to an 8 × 8 Latin square design with the 8 diets and eight 7-d periods. Thus, each pig was fed each diet in one period and no pig received the same diet more than once. Pigs were limited to 3 times the maintenance requirement for metabolizable energy (i.e., 90 kcal ME per lb BW 0.60 ; NRC 6 ). Through the experiment, pigs had free access to water. Pigs were deprived of feed overnight at the end of each experimental period, to be fed with a new diet the following morning. Each period lasted 7 days with the initial 5 days being the adaptation period, and ileal digesta was collected for 9 hours on d 6 and 7. All collected digesta samples were lyophilized and analyzed for crude protein. Amino acid values and standardized ileal digestibility (SID) were calculated as described in Stein et al. 7 Dietary treatments were arranged in a 2 × 2 × 2 factorial with main effects of crystalline amino acid level (AA; low vs. high); reducing sugars (RS; low vs. high); and diet form (mash vs. pellet; Table 1). For crystalline amino acid treatments, diets were considered low or high based on the inclusion of crystalline AA with high crystalline AA diets having increased concentrations of lysine, threonine, and tryptophan compared with low crystalline AA diets. Valine and isoleucine were included as needed in the high crystalline AA diets. Reducing sugars were naturally occurring in ingredients (corn and soybean meal-based diets; low) or increased by adding DDGS and bakery meal (20 and 15%, respectively; high). All diets contained 0.5% titanium dioxide as an indigestible marker to allow for calculation of AID and SID of AA as described below.

Chemical Analysis
Representative samples of corn, bakery meal, DDGS, soybean meal and treatment diets were analyzed at the University of Missouri Agricultural Experiment Station Chemical Laboratory (Columbia, MO) for dry matter, 8 crude protein, 9 crude fat, 10 crude fiber, 11 ash, 12 complete AA profile, 13 available Lys, 14 and protein solubility in potassium hydroxide (KOH) 15 (Tables 2 and 3). Ileal digesta contents were analyzed for dry Swine Day 2020 matter, 8 crude protein, 9 and complete AA profile. 13 Diet and ileal digesta samples were also analyzed for titanium dioxide. 16 Diet manufacture occurred at the Kansas State University O.H. Kruse Feed Technology and Innovation Center (Manhattan, KS). Whole grain ingredients were ground with a three-high roller mill (RMS Model 924) to an approximate particle size of 600 microns. Feed was mixed in a 1-ton Hayes and Stolz double ribbon mixer. Treatments were pelleted using a 5-ton 100-horsepower pellet mill (Model PM3016-4, California Pellet Mill) equipped with a 5/32 × 1 3/8 in die (L:D = 8.75). The conditioning temperature ranged from 175 to 185°F to achieve a hot pellet temperature of 185 to 190°F and was attained by adjusting the steam addition (Table 4). The pelleted diets were cooled in an experimental counterflow cooler for 15 minutes. To minimize the effect of pellet quality differences, pellets passed through a sifter to remove fines before transport.
A sample of cool pellets was collected, and the fines sifted off by using the corresponding sieve stack. 17 Sifted pellets were split using a riffle divider and 100 grams used for analysis. The 100-g sample was placed into the hopper of the Holmen NHP100 for 60 seconds. The fines were removed as the sample was run. Once completed, the sample was removed from the hopper and weighed. The PDI was calculated by dividing this final sample weight by the 100-g initial sample weight.

Calculations and Statistical Analysis
Values for SID (%) were calculated as indicated below using Stein et al. 6 : where AA digesta and AA diet represent the AA concentrations (g/kg) in digesta and diet dry matter (DM), respectively, and Ti diet and Ti digesta represent the digestible marker concentrations (g/kg) in diet and digesta DM, respectively.
where AID AA was previously calculated in equation 1, IAA end represents basal endogenous losses where average values from digestibility experiments at the University of Illinois were used, and AA diet represents the AA concentrations (g/kg) in diet DM.
Data were analyzed as a completely randomized Latin square design using the GLIMMIX procedure of SAS (v. 9.4, SAS Institute, Inc., Cary, NC) with pig as the experimental unit. Fixed effects included feed form, crystalline AA, and RS inclusion; and the interaction between all three, feed form and crystalline AA, feed form and RS, and crystalline AA and RS inclusion. Least square means were calculated for each Swine Day 2020 independent variable and means were separated using the PDIFF option. Results were considered significant at P ≤ 0.05.

Results
Chemical analysis of experimental diets indicated that analyzed CP ranged from 13.3 to 19.3% with the expected range of 14.8 to 19.1% for both mash and pelleted diets. The high crystalline AA × low RS diets contained the lowest CP, and the low crystalline AA × high RS diets contained the highest CP (Table 3). The analyzed protein solubility in potassium hydroxide (KOH) values ranged from 48.62% in the low crystalline × high RS mash diet to 81.76% in the low crystalline AA × low RS pelleted diet. Analyzed available lysine content was 0.85% to 1.02%, where the lowest was high crystalline AA × high RS mash diets and highest for the low crystalline AA × low RS mash diet. The Lys:CP expected values ranged from 5.18 to 6.31, where the lowest ratio (5.83) was the high crystalline AA × low RS mash and pelleted diets, and the highest ratio (7.04) was in the low crystalline AA × high RS pellet diet.
The experiment was designed to pellet diets at a conditioning temperature of 190°F. However, at the desired conditioning temperature a consistent pelleting run could not be achieved. Therefore, diets were pelleted to a target hot pellet temperature between 185 and 190°F, which was achieved by pelleting at a conditioning temperature range of 175 to 185°F (Table 4). The PDI for diets containing low RS were improved by more than 10% compared with those containing high RS. These differences in PDI could possibly be explained by increases in added fat in diets containing high RS. Differences in PDI were alleviated by sifting the pellets after post pelleting to remove excessive fines.
There was no feed form × crystalline AA × RS interaction observed for SID of AA. There were no 2-way interactions of feed form × crystalline AA for SID of total AA, indispensable, or dispensable AA (Table 5). There were no 2-way interactions of feed form × RS observed for SID of total AA, CP, or dispensable AA (Table 6). There were no 2-way interactions of feed form × RS observed for SID of indispensable AA, except for tryptophan. Pigs fed pelleted high RS diets resulted in lower SID of tryptophan than mash high RS diets, mash low RS diets, and pelleted low RS diets.
There were no 2-way interactions of crystalline AA × RS observed for SID of total AA, CP, indispensable AA, and dispensable AA (Table 7).
For the main effects of feed form, the SID of total AA, CP, indispensable AA, alanine, aspartic acid, glutamic acid, and serine increased (P < 0.042) in the pelleted diets compared with mash diets (Table 8). For the main effects of crystalline AA, pigs fed low or high crystalline AA had SID of total AA and CP that were not different. Pigs fed high crystalline AA had increased (P = 0.007) SID of tryptophan compared with those fed low crystalline AA diet. The SID of lysine tended to increase (P = 0.076) in pigs fed high crystalline AA diets compared with those fed low crystalline AA diets. Pigs fed high crystalline AA had decreased (P = 0.050) SID of histidine compared with those fed low crystalline AA diets. The SID of arginine and isoleucine tended to decrease (P < 0.079) in pigs fed high crystalline AA. In pigs fed high crystalline AA the SID of serine and glycine decreased (P < 0.042) compared with those fed low crystalline AA. For the main effects of RS diets, pigs fed high RS diets had decreased (P < 0.05) SID

Conclusion
This experiment was designed to determine if pelleting different swine diets influence the digestibility of amino acids. Diets were formulated with low or high crystalline AA and low or high reducing sugars provided by co-product ingredients, DDGS, and bakery meal to increase the chances of binding lysine via the Maillard reaction. Diets were pelleted to a target hot pellet temperature between 185 and 190°F. There was no evidence of interactions between diet types, indicating that increasing amounts of crystalline AA and RS did not increase the Maillard reaction or reduce AA digestibility when pelleting diets by using the reported conditions. In addition, pelleting diets resulted in improved AA digestibility. Diets formulated with 20% DDGS and 15% bakery (high RS) resulted in decreased AA digestibility compared with the cornsoybean meal-based diets.    Swine Day 2020